molecules Article Comparison of Volatiles in Different Jasmine Tea Grade Samples Using Electronic Nose and Automatic Thermal Desorption-Gas Chromatography-Mass Spectrometry Followed by Multivariate Statistical Analysis 1,2, 1,3, , 4 1,2 1,2, Shuyan Wang y, Feng Zhao * y, Wenxi Wu , Pengjie Wang and Naixing Ye * 1 College of Horticulture, Fujian Agriculture and Forestry University, Fuzhou, Fujian 350002, China; [email protected] (S.W.); [email protected] (P.W.) 2 Key Laboratory of Tea Science in Fujian Provincial University, Fujian Agriculture and Forestry University, Fuzhou, Fujian 350002, China 3 College of Pharmacy, Fujian University of Traditional Chinese Medicine, Fuzhou, Fujian 350122, China 4 Hengzheng Testing Technology Co., Ltd., Fuzhou, Fujian 350100, China; [email protected] * Correspondence: [email protected] (F.Z.); [email protected] (N.Y.) These authors contributed equally to this work and should be considered co-first authors. y Received: 23 November 2019; Accepted: 14 January 2020; Published: 16 January 2020 Abstract: Chinese jasmine tea is a type of flower-scented tea, which is produced by mixing green tea with the Jasminum sambac flower repeatedly. Both the total amount and composition of volatiles absorbed from the Jasminum sambac flower are mostly responsible for its sensory quality grade. This study aims to compare volatile organic compound (VOC) differences in authoritative jasmine tea grade samples. Automatic thermal desorption-gas-chromatography-mass spectrometry (ATD-GC-MS) and electronic nose (E-nose), followed by multivariate data analysis is conducted. Consequently, specific VOCs with a positive or negative correlation to the grades are screened out. Partial least squares-discriminant analysis (PLS-DA) and hierarchical cluster analysis (HCA) show a satisfactory discriminant effect on rank. It is intriguing to find that the E-nose is good at distinguishing the grade difference caused by VOC concentrations but is deficient in identifying essential aromas that attribute to the unique characteristics of excellent grade jasmine tea. Keywords: Chinese jasmine tea; tea grade; volatile organic compounds; electronic nose (E-nose); automatic thermal desorption-gas-chromatography- mass spectrometry (ATD-GC-MS); multivariate statistical analysis Chemical compounds studied in this article: methyl salicylate (PubChem CID: 4133), linalool (PubChem CID: 6549), linalool oxide (furanoid) (PubChem CID: 240), ethyl decanoate (PubChem CID: 8048) 1. Introduction It is a general belief that the pleasant aroma of the Jasminum sambac flower can relieve the mood of depression [1]. Moreover, the health effect of tea also has been widely confirmed [2]. Both of these concepts make jasmine tea a popular tea worldwide [3,4]. Traditionally, the processing of jasmine tea, includes the following seven steps (shown in Figure S1) of tea dhool preprocessing, fresh flowers maintenance, tea and flower combination, scenting, flower removal, drying, and packing [5]. Current Chinese National Standards subdivide jasmine tea into six grades according to the number of times of repeated scenting which affects the quality of both the Jasminum sambac flower and the tea dhool [6,7]. The floral fragrance adsorption and persistence are critical factors related to jasmine tea grading [8]. Molecules 2020, 25, 380; doi:10.3390/molecules25020380 www.mdpi.com/journal/molecules Molecules 2020, 25, 380 2 of 16 Presently, there are existing studies on the evaluation of jasmine tea quality. Chen et al. observed the changes of volatile compounds during the scenting processes, and marked a serial of positive correlated compounds [3], for example. Lin et al. proposed a jasmine tea flavor (JTF) index (the ratio of peak area percentage of (Z)-3-hexenyl benzoate, α-farnesene, methyl anthranilate to linalool) as a novel quality evaluation index for jasmine tea’s volatile organic compound (VOC) evaluation [9]. Shen et al. believed the adsorption and retention of endogenous volatiles of tea was key for its quality [8]. Liang et al. analyzed the application of chemical composition and solution color to the difference of jasmine tea in its quality evaluation [10]. Electronic nose (E-nose) is another technique which has been widely used in product quality testing [11], medical diagnosis [12] and environmental monitoring [13], for example. E-nose can make a simple, fast and effective discrimination [14,15]. Its vital module is the sensor array of metal oxide films which can simulate the human nose and generate corresponding signals for gases. The response value of the e-nose is R/R0. R0 is the reference resistance obtained by cleaning the electronic nose before testing, and R is the sample resistance obtained during testing. While, E-nose is also a typical gray box system, which mainly constructs the discriminant model between input signals and output results through algorithm training [11], it means that, although the correct judgment could be given, it is still hard to tell which substances play a key role in grade contingencies. Gas-chromatography- mass spectrometry (GC-MS), coupled with an enrichment pretreatment is the most commonly used method. Conventional existing enrichment methods include simultaneous distillation and extraction [16], headspace solid-phase microextraction [17], solid-phase extraction [18], accelerated solvent extraction [19] and more. Automatic thermal desorption (ATD) is a new prominent enrichment method, which has the advantages of convenient operation, a high enrichment rate, good reproducibility and no use of organic solvents. The combination of ATD to GC-MS has been used in air monitoring [20], analysis of pesticides in the atmosphere [21], material and emission analysis [22], food and aroma analysis [23] etcetera. The main advantage of applying ATD to the detection of jasmine aroma is the content of VOC enrichment could be much higher than that found by solid-phase microextraction (SPME). The purpose of this study is to compare aroma characteristics within different jasmine tea grade samples through distinct techniques. A group of corresponding samples are subjected to research. Both electronic nose and automatic thermal desorption-gas-chromatography-mass spectrometry (ATD-GC-MS) are applied. Their discriminant effects are compared systematically. Accordingly, the rapid classification of jasmine tea is achieved using an electronic nose, while ATD-GC-MS detection followed by multivariate data analysis can provide a more profound understanding of the composition of volatile substances related to grading classification. 2. Materials and Methods 2.1. Sample Information A group of authoritative jasmine tea grade samples (including six grades, indicated as 1G, 2G, 2G, 3G, 4G, 5G and 6G, three repeats per grade for automatic thermal desorption-gas-chromatography-mass spectrometry (ATD-GC-MS) tests, six repeats per grade for the E-nose test) prepared according to Chinese National Standards GB/T 34779-2017 [5], were provided by Fujian Tea Import and Export Company Limited. (Fuzhou city, Fujian province, China). All samples were stored in a refrigerator at 4 ◦C before analysis. 2.2. Chemicals Volatiles standards, including methyl salicylate (PubChem CID: 4133; 99.5%), linalool (PubChem CID: 6549; 99.5%), linalool oxide (furanoid) (PubChem CID: 240; 99.5%), ethyl decanoate (PubChem ≥ ≥ CID: 8048; 99%), were purchased from Aladdin (Shanghai, China). ≥ Molecules 2020, 25, 380 3 of 16 2.3. Automatic Thermal Desorption-Gas-Chromatography-Mass Spectrometry (ATD-GC-MS) Analysis The volatile organic compound (VOC) of jasmine tea samples was analyzed using an ATD-GC-MS method, described by Zheng [24], with slight modification. A COLIN Tech Auto thermal desorption sampler (Chengdu Colin Analytical Technology Co., Ltd., Chengdu, China) and a Shimadzu 2010 gas-chromatography (GC) coupled with 8040 triple quadrupole mass spectrometry (TQ-MS) (Shimadzu Production Institute, Kyoto, Japan) was applied. 2.3.1. Extraction of Volatile Organic Compounds (VOC) A QC-1S atmosphere sampling instrument (Beijing Kean Labor Insurance New Technology Co., Ltd., Beijing, China) was used for VOC extraction according to China’s National Environmental Protection Standards [25]. The VOC analysis method was the same as Zheng et al. [24]. Briefly, 3.0 g of sample was weighted into a headspace bottle and ethyl decanoate (100 ppm, 15 µL) was added to the samples as the internal standard. Then, the headspace bottle was sealed and equilibrated at 55 ◦C for 20 min. Afterward, the sorbent tube (Chengdu Colin Analytical Technology Co., Ltd., Chengdu, China) was connected to the atmosphere sampling instrument and headspace bottle according to the flow direction of the sorbent tube with polytetrafluoroethylene (PTFE) pipes. Finally, volatile components were collected at 200 mL/min flow rate for 30 min. After sample collection, both ends of the sorbent tube were sealed with PTFE caps and transported to the laboratory for analysis. 2.3.2. Thermal Desorption Thermal desorption was conducted by a COLIN Tech Auto thermal desorption sampler (Chengdu Colin Analytical Technology Co., Ltd., Chengdu, China). The primary thermal desorption of sampling tube was carried out at 250 ◦C for 5 min. To introduce trapped compounds into the gas chromatograph, the cold trap was then heated rapidly from 25 C to 300 C. The temperature of the valve and transfer − ◦ ◦ line were maintained at 200 ◦C during analysis. Then, the whole system was baked at 300 ◦C for 3 min in preparation for the next sample analysis. 2.3.3. Gas-Chromatography-Mass Spectrometry Analysis Volatile organic compounds were identified using a 2010 GC coupled with an 8040 TQ-MS system (Shimadzu Corporation, Kyoto, Japan). The capillary column was a Shimadzu Rtx-5MS capillary column (30 m 0.25 mm 0.25 µm), and the carrier gas was helium at 1.0 mL/min. The split ratio was × × 1:40. The inlet temperature was 240 ◦C. The gradient temperature program was as follows: initial oven temperature was 40 ◦C, held for 3 min; 40–120 ◦C at 5 ◦C/min, held for 5 min; 120–240 ◦C at 30 ◦C/min, held for 8 min. The ionization mode of the MS was electron impact (EI). The temperatures of the interface and ion sources were 280 ◦C and 230 ◦C, respectively.